scsi hard drive changed by sta hard drive

hi friends,
please i just join this forum,i don t have a lot experience with computers maintenance.I have a dell unit with a scsi hard drive.my hard drive damged (got medium error).I tried to change it by an sata hard drive.I restored the image of my unit on the sata hard drive(i connected the sata hard drive on the motherboard and i did the restore image with success).i tried to boot the computer from the sata hard drive but I could not.I looked on the bios boot sequence I found that the onboard sata hard drive is not preceded by a number.so i can not boot from the sata hard drive.why during the back up the sata hard drive was detected and when i try to boot from it,it was not detected.how to fix this problem/
thanks